cluster/afr: if a subvolume is down wind the lock request to next

When one of the subvolume is down, then lock request is not attempted
on that subvolume and move on to the next subvolume.

/* skip over children that are down */
                while ((child_index < priv->child_count)
                       && !local->child_up[child_index])
                        child_index++;

In the above case if there are 2 subvolumes and 2nd subvolume is down (subvolume
1 from afr's view), then after attempting lock on 1st child (i.e subvolume 0)
child index is calculated to be 1. But since the 2nd child is down child_index
is incremented to 2 as per the above logic and lock request is STACK_WINDed to
the child with child_index 2. Since there are only 2 children for afr the child
(i.e the xlator_t pointer) for child_index will be NULL. The process crashes
when it dereference the NULL xlator object.

Change-Id: Icd9b5ad28bac1b805e6e80d53c12d296526bedf5
BUG: 765564
Signed-off-by: Raghavendra Bhat <raghavendra@redhat.com>
Reviewed-on: http://review.gluster.org/4438
Reviewed-by: Krishnan Parthasarathi <kparthas@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>